Heydo That's true, but we also don't measure galaxy rotation by watching things move. The rotation rate of galaxies is incredibly slow on human timescales. Like, the Milky Way rotates once every 100,000 - 150,000 years.

Instead, we're looking at differences in redshift from one side of the galaxy compared to the other. A rotating object -- assuming it's axis of rotation isn't perfectly facing you (i.e. you're looking straight down its north/south pole) -- always has one side that's moving toward you, and another that is moving away from you. The side that is moving toward you will be slightly blueshifted compared to the average, while the side that is moving away will be slightly redshifted. Finding a galaxy that isn't rotating means finding a galaxy that doesn't have this red/blue shift pattern.